Simple reader monad

gistfile1.hs

instance Functor ((->) t) where
    --fmap :: (a -> b) -> (t -> a) -> (t -> b)
    fmap = (.)

instance Monad ((->) t) where
    return = const
    --(>>=) :: (t -> a) -> (a -> t -> b) -> t -> b
    f >>= g = \t -> let a = f t
                    in g a t

data Person = Person { name :: String, age :: Int, address :: String }

showPerson :: Person -> String
showPerson = do
    x <- ("Name: " ++) . name
    y <- ("Age: " ++) . show . age
    z <- ("Address: " ++) . address
    return . unlines $ [x,y,z]

With Control.Monad.Reader:

showPerson4 :: Person -> String
showPerson4 = 
    let showPersonReader = do
        x <- asks $ ("Name: " ++) . name
        y <- asks $ ("Age: " ++) . show . age
        z <- asks $ ("Address: " ++) . address
        return . unlines $ [x, y, z]
    in runReader showPersonReader